Zinc oxide is a white powder with a hexagonal crystal structure, and in recent years it is being used for a variety of purposes, in the formation of thin-films. The methods used to form such zinc oxide thin-films include physical vapor deposition methods such as vacuum vapor deposition and sputtering, and chemical vapor deposition methods such as CVD. Sputtering methods are widely employed as methods for forming zinc oxide thin-films, because they allow stable film formation even with low-vapor-pressure materials, and the process is simple.
Sputtering is a method in which a positive ion such as argon ion is physically impacted with a target set on a cathode, the impact energy causing the material of the target to be discharged, accumulating a film of approximately the same composition as the target material on a substrate that has been set facing the target. Sputtering methods include direct current sputtering methods (DC sputtering methods), and radio-frequency sputtering methods (RF sputtering methods). Direct current sputtering methods involve using a DC power supply to apply a direct-current voltage to the cathode target, and they have the advantage of high film-forming speed and high productivity. However, direct current sputtering methods are restrictive in that the resistivity of the target used must be no greater than 105 Ω·cm. This is because using a target with a resistivity of 105 Ω·cm or greater results in non-stable discharge being generated during sputtering.
On the other hand, high-frequency sputtering methods employ a high-frequency power source instead of a DC power supply. Although the target used does not have to be a conducting material, the film deposition rate is slower than with a direct current sputtering method, and the productivity tends to be low. Also, because of the complexity and high cost of the power source and apparatus, equipment cost also tends to be high. A demand therefore exists for a target that can be used in direct current sputtering methods.
Zinc oxide thin-films are typically used as transparent conductive films. For example, zinc oxide thin-films containing zirconium added at 0.1 atomic percent (corresponding to 1120 ppm by weight) or greater have been proposed, which are imparted with conductivity by addition of additives, for use as transparent conductive films for solar cells (PTL 1, for example). However, the electric conductivity of such thin-films is 0.0003 Ω·cm, and they cannot be used as high-resistance thin-films.
Other uses of thin-films composed mainly of zinc oxide are for zinc oxide thin-films with high resistance. For example, the use of zinc oxide thin-films with high resistance values of 1.0 Ω·cm or greater as buffer layers in CIGS thin-film solar cells has been proposed (PTL 2, for example). For such purposes, the resistivity of the zinc oxide thin-film is preferably as high as possible.
The zinc oxide targets used to form such zinc oxide thin-films by sputtering are of high purity, containing absolutely minimal additives or impurities (PTL 3, for example). The reason for this is that the presence of impurities in the film generates carrier electrons from the impurities which lower the resistivity of the film, making it impossible to obtain a thin-film with high resistance (PTL 2).
A zinc oxide thin-film having such high resistance can be formed by sputtering using relatively inexpensive starting materials, but two problems arise in this case.
The first problem is that the zinc oxide target used to form such a thin-film has low conductivity and is therefore unsuitable for direct current sputtering methods. A high-purity zinc oxide target has low impurity-driven carrier generation, and therefore has resistivity of 107 Ω·cm or greater. It is therefore unsuitable for direct current sputtering methods. It is therefore necessary to employ high-frequency sputtering methods, which are associated with low film-forming speeds and high equipment cost.
Reactive sputtering is a known strategy for avoiding this problem. The method is one in which argon gas is introduced together with a reactive gas, and the target material discharged by the impact of argon ions reacts with the reactive gas, accumulating as an insulating film on the substrate. It is a feature of the method that it allows formation of insulating films by direct current sputtering using conductive targets. For formation of a high-resistance zinc oxide thin-film, using a conductive metal zinc target and adding oxygen gas to the argon gas for direct current sputtering allows an insulating zinc oxide thin-film to be accumulated. In this method, however, the quality of the zinc oxide thin-film varies significantly depending on the amount of reactive gas added and changes in the zinc deposition rate over time. Control of the reaction is therefore difficult, and it is difficult to obtain a film with stable resistivity.
Another method of forming a zinc oxide thin-film with high resistance has been proposed, in which zinc metal is mixed with zinc oxide powder and firing is conducted at below the melting temperature of zinc (PTL 5). Since the target in this method contains zinc metal, it is possible to lower the resistance. However, since the firing temperature is near the melting point of zinc (419.5° C.), densification of zinc oxide does not progress sufficiently. The target is therefore quite brittle, and the target readily cracks during sputtering.
The second problem in forming a zinc oxide thin-film with high resistance by sputtering is the low strength of the target. Here, “strength” means the physical strength of the target, and the measured value is represented as the transverse intensity. Virtually no impurities are present in a high-purity zinc oxide target. When a target is produced by firing, therefore, there is minimal sintering inhibition by impurities. As a result, although grain growth of the target proceeds, abnormal grain growth also takes place, drastically lowering the strength of the sintered compact. In addition, since the exterior of the sintered compact undergoes sintering and densification before the interior, air bubbles remain in the interior and the density of the sintered compact tends to be reduced. Furthermore, the virtual lack of impurities weakens interaction at the grain boundaries, and cracking, chipping and shedding very frequently take place at the grain boundary sections.
In order to ameliorate this problem, it has been proposed to conduct firing of zinc oxide at a high temperature of 1200° C. so that densification occurs through to the interior of the sintered compact (PTL 3). However, firing at such a high temperature results in accelerated grain growth of the sintered compact and increased particle size. As a result, the transverse intensity of the obtained target is less than 40 MPa and it is extremely brittle. The low transverse intensity of such a target tends to lead to cracking of the target in sputtering methods.